In a strapdown inertial system the gyros are fixed to the aircraft and subjected directly to the motion of the aircraft. Normally the average effects of the input angular rates about the input gyro axis are compensated for in the system computer. Angular rates about the gyro spin axis will increase or decrease the absolute spin velocity of the gyro and thereby change the scale factor of the gyro torquer. This is caused by the fact that the gyro is driven by a synchronous motor excited by a constant frequency. This effectively locks the speed of the gyro to the airframe. Aircraft angular rates about the spin axis require that the gyro wheel be accelerated or decelerated to keep up with the aircraft movement. The computer compensation circuits attempt to correct the torquer scale factor changes caused by the average changes in the absolute gyro wheel speed.
The compensation made in the computer is not dynamically correct whenever there is angular vibration or angular acceleration displaced from the gyro spin axis. The problem is that the synchronous motor is essentially a torsion spring whose torque is approximately equal to K.theta. where .theta. is the phase angle between the stator motor flux and the magnetic pole of the rotor. This tortional spring in combination with the inertia of the gyro wheel forms a torsion pendulum whose period is usually a few cycles per second. The gyro speed will then "hunt" about the synchronous speed when subjected to angular perturbations. In a strapdown inertial system this gyro wheel hunting may give rise to unpredictable scale factor errors which are not compensated for in the computer.
Normally strapdown systems compensate for the change in scale factor of the torquer by assuming that the gyro speed with respect to the mounting body is a constant. The computer, by using the measured angular rates, then computes the change in torquer scale factor of the torquer based on this premise. Since the torquer capability of the gyro spin motor is low its ability to accelerate and decelerate the rotor is poor. Therefore, under angular accelerations and angular vibration the actual action of the wheel momentum is unknown and we therefore introduce errors.
The present invention overcomes these scale factor errors by devising a device that will adjust the frequency of the gyro wheel supply in such a manner as to maintain the speed of the gyro inertially constant. If the frequency of the gyro wheel supply is properly modulated by a signal proportional to the angular rate about the gyro spin axis, there will be no acceleration of the gyro synchronous motor output. Therefore, the torsion pendulum will not be disturbed and the gyro will run at a constant inertial speed and no torquer compensation will be required due to spin axis rate.